In recent years, demand for portable terminals such as a cellular phone and a lap-top computer has grown. Particularly, thin portable terminals having sound function such as a video phone, a movie play, and a hands-free phone function as commodity values have being developed. In the development thereof, the requirement for a small-sized and high-output electro-acoustic transducer has increased. In electronic devices such as a cellular phone, an electro-dynamic electro-acoustic transducer has been used as an electro-acoustic transducer. The electro-dynamic electro-acoustic transducer is composed of a permanent magnet, a voice coil, and a vibrating membrane. However, the electro-dynamic electro-acoustic transducer has a limit to a reduction in thickness due to the operation principle and the structure thereof. Consequently, for example, as disclosed in Patent Documents 1 to 3, it is expected to use a piezoelectric vibrator as an electro-acoustic transducer. In particular, Patent Document 3 discloses a parametric speaker configured with the piezoelectric vibrator.
In addition, as disclosed in Patent Document 4, for example, there is a sound wave sensor as a use of the piezoelectric vibrator. The sound wave sensor is a sensor that detects the distance to an object or the like using a sound wave oscillated from the piezoelectric vibrator, or the like.